The primary aim of this study is to examine the effect of the consensus public health recommended level of physical activity [moderate physical activity (MOD-PA) = 150 min/wk] versus a higher dose of physical activity [high physical activity (HIGH-PA) = 250 min/wk] in the context of a comprehensive behavioral weight loss program that a includes a reduction in energy intake measures of cardiovascular function using cardiac MRI and biomarkers of vascular disease risk. Each of these exercise doses will be compared independently to a Diet Only group intervention on the proposed primary and secondary outcomes, with MOD-PA also compared to HIGH-PA. This study involves the recruitment of 390 overweight and obese adults who will be randomly assigned to one of the above conditions (Diet Only, MOD-PA, HIGH-PA) for a period of 12 months. Assessment of the primary and secondary outcomes will occur at baseline prior to participation in the intervention, following the initial six months of the intervention, and at the conclusion of 12 months of the intervention. The primary outcome is LVM measured by cardiac MRI. Secondary outcomes include additional cardiac MRI measures (aortic pulse wave velocity, end diastolic volume, aortic distensibility), inflammatory markers (CRP and TNF1) and selected adipocytokines (adiponectin) as biomarkers of risk related to vascular outcomes, body weight, body composition, and cardiorespiratory fitness, and traditional CVD risk factors (lipids, glucose, insulin, blood pressure). Additional secondary analyses will allow for examination of the effects of physical activity independent of weight change on the primary and secondary outcomes.